Final answer:
The correct enthalpy of formation reaction for solid CaO is the reaction in which one mole of Ca(s) reacts with ½ mole of O₂(g) to produce one mole of CaO(s): 2Ca + O₂ → 2CaO.
Step-by-step explanation:
The correct enthalpy of the formation reaction for solid CaO is represented by the equation:
Ca(s) + ½ O₂(g) → CaO(s)
This equation indicates that one mole of calcium oxide (CaO) is formed from one mole of solid calcium (Ca) and half a mole of gaseous oxygen (O₂) under standard conditions. The formation reaction must produce exactly one mole of the compound from its constituent elements in their most stable forms at standard state, which for carbon is graphite and for oxygen is O₂ gas. The option that correctly represents the formation of CaO from its elements is option 4: 2Ca + O₂ → 2CaO because it reflects the production of one mole of CaO as the balanced equation shows the formation of two moles of CaO when two moles of calcium react with one mole of oxygen.